1. Field of the Invention
This invention relates to an improvement in the process for the formation of fibrils from high molecular weight polymers. More particularly, the subject invention pertains to an improvement in the method of making fibrils from high molecular weight polyolefins, in particular polyethylenes and polypropylenes, whereby the resulting paper on other sheet-like structures into which these fibrils or fibril material are incorporated are improved. The fibrils produced by this invention, and particularly when produced employing the improvement in the process thereof, are capable of forming coherent self-supporting water leaves which can be used for the production of sheet-like structures according to known methods of paper manufacturing which are of improved quality.
2. Description of the Prior Art
In Davis et al, Ser. No. 193,716, filed Oct. 29, 1971, entitled "Improved Fibril and Process"; Davis et al, Ser. No. 202,302, filed Nov. 26, 1971, entitled "Improved Fibril Process"; and Davis et al, Ser. No. 211,562, filed Dec. 23, 1971, entitled "Fibril Process", there has been disclosed a number of processes for producing high quality fibrils which are especially suitable for and readily adaptable to incorporation into paper or other sheet-like structures which are manufactured by the known paper-making processes. The invention disclosed herein is an improvement in those processes.
In each of these previously mentioned inventions, the fibrils are produced from a solution of a very high molecular weight polyolefin, particularly polyethylene or polypropylene, in which the solution is sheared or subjected to a shearing action whereby the polymer molecules therein are oriented and immediately thereafter the polymer solute is made to precipitate from the solution by purely thermal means, which is attained by rapidly lowering the temperature, as in a quenching bath or by other cooling means. As has been previously disclosed, the most useful systems for this type of fibril production are those systems employing polyolefins and suitable liquid hydrocarbon solvents for these polyolefins. The cooling or quenching of these oriented solutions is usually carried out under conditions of zero shear and at temperatures well below the precipitation temperatures of the polymer solutions to result in the formation of solvent swollen fibrous masses. The solvent swollen fibrous masses are then normally converted into fibrils by a series of subsequent operations which usually include the removal of excess solvent from the fibrous mass, the cutting of the fibrous mass into pieces of desired length, and the beating and refining of the cut or chopped fibrous mass into individual fibrils for use in the production of paper or other sheet-like materials on paper-making machinery by the methods normally employed in the production of paper.
While the paper hand sheets fabricated in each of the hereinabove identified disclosures from a plurality of the fibrils produced by the process of each of the respective inventions were of good quality, it was observed that some of the sheet-like structures were not completely smooth. Those paper hand sheets which were not completely smooth seemed to contain small, miniscule knot-like lumps of varying sizes that protruded from the surface of the sheet. This condition, i.e., containing small lumps or knots of various sizes, was more visually apparent upon holding the paper hand sheets up to a strong light and observing the same. It was then hypothesized that the lumps or knots in the resultant paper sheets were caused by particles of incompletely dissolved polymer in the starting polymer solution from which the fibrils were obtained. Therefore, the original or starting polyolefin/hydrocarbon solvent solutions from which the fibrils were produced were checked and examined prior to the beginning of the fibril formation process. This examination did not result in an apparent answer or give any clear-cut evidence as to the fact that undissolved polymer might be present in the solutions and result or eventually show up as knots or lumps in the paper hand sheets fabricated from the fibrils produced from the polymer solutions.